Using Soil Temperature as a Success Strategy for Northern Gardening

Early February typically marks the kick-off of my workshop season, with "Seed-Starting Success for the Northern Gardener" as the first class of the year. This year it was offered in cooperation with a local garden center, and attendance was considerably greater than I expected, which was really wonderful! I felt super encouraged at the level of interest in this topic, and was pleased that there were so many keen learners in the group.

We covered a ton of content over the three hours that we were together, and as we explored some of the handouts that I had provided them with, we spent a lot of time exploring the Seed-Starting Schedule chart that I've developed specifically for our area. In past seed-starting workshops I have had to reiterate the fact that these schedules really do require interpretation through a lens of good old common sense, and that no matter how well the schedule is adapted to our area, if outdoor conditions are not amenable to seeding or planting out your pre-started seedlings, you need to delay and wait until the time is favorable.

Understanding the extreme variability of our regional weather patterns, it makes sense that we need to build flexibility into our gardening schedules. Through the many workshops that I've offered over the years I have awakened to the fact that most folks who are just starting out their food-growing adventures have a natural tendency towards wanting to follow an established template. They have a sincere desire to be successful in their efforts, and feel that if there is some tried-and-true, no risk, fail-safe schedule for seeding and planting, then they can feel very comfortable "buying in" to the process.

When I think of how many years I have been growing food, and how long it took me to arrive at the level of comfort regarding seasonal wisdom, (which, by the way, included lots of failures) I realized that I could offer my students an additional leg-up to really assist them in achieving success when it comes to seeding and planting-out in their gardens instead of just offering them a "template". One of the resources I utilize is information regarding minimum and optimum soil temperatures for direct seeding out into the garden as well as minimum and ideal temperatures for transplanting our pre-started seedlings.

With this information, combined with the Seed-Starting Schedule, my students now have the tools they need to help them flex those "common-sense" muscles so vital to northern gardening success. It very importantly gives them the opportunity to exercise discernment regarding when to start seeding, based on more than just a schedule or planting template. Gardening successfully in our crazy climate requires that we consider not only the average planting dates, but also whether soil and weather conditions are actually amenable. Like all systems, building resilience into our seeding and planting systems is absolutely vital no matter where we live, and having some strategies that help us build that in is invaluable.

Another reason why soil temperatures are an important consideration is that at certain temperatures various soil microorganisms become active. Once germination has been achieved, the seedling has, for the most part, used up the bulk of the seeds stored energy, and it starts scavenging for soil nutrients to fuel its next stage of growth. If the soil is too cool (or too warm for some crops), not only will germination be hindered, but the microbial associations for nutrient uptake may not actually be available. Seeding (or transplanting) into ideal soil temperatures for your crop provides optimal conditions for your plants to not only survive, but thrive.

So let's take a look at some of those soil temperature requirements for seeding.

The chart below outlines some of the most common vegetables that we will sow directly into the garden in our northern climate, and what their seeding temperature requirements actually are.


ARUGULA (direct sow) 10 C 10 C

BEANS (direct sow) 18 C 21 C

BEETS (direct sow) 10 C 12 C

CARROTS (direct sow) 7 C 14 C


LETTUCES (direct sow) 10 C 16 C

MACHE (direct sow) 5 C 14 C

PARSNIPS (direct sow) 12 C 15 C

PEAS (direct sow) 5 C 10 C

RADISH (direct sow) 10 C 12 C

RUTABAGA/TURNIP (direct sow) 10 C 15 C

SPINACH (direct sow) 5 C 12 C

SWISS CHARD (direct sow) 10 C 12 C

Most garden centers now sell a variety of soil thermometers at various price points, and it's my opinion that they are incredibly useful tools for providing accurate information to help you decide when the time is right to seed your outdoor crops. Once spring is imminent, get out there with your soil thermometer and check out what's going on in various areas of your property. If you really want to geek out, make a chart that includes various areas in your landscape. You can gain some valuable insight about your property, including it's varied microclimates, by recording soil temperatures in these areas on a weekly basis in the spring. This will provide you with information about which are